It is more difficult to specify and give examples of the minimum requirements to read FITS formats because reading FITS means a translation of the FITS data set into the internal format of a data analysis package. Each package has its own unique requirements. When developing software to read FITS format, allow for the flexibility of FITS. Don't assume that files will always follow practices that are recommended but not required. For example, don't expect that values of optional keywords will use the recommended, but not mandatory, fixed format; be prepared to accept all formats consistent with FORTRAN-77 list-directed read. Also, assume that keywords will appear in a particular order only when this order is specified by the rules of FITS. Because values of keywords may contain lower case letters, comparisons should not be case sensitive.
A FITS file may contain records after the end of the FITS primary HDU and the extensions. Such special records can be recognized by the fact that they begin where FITS would begin an extension, at the end of the previous extension or the primary HDU, but the first 8 bytes do not contain the string ``XTENSION.'' While the FITS rules do not forbid the string ``SIMPLE '', its presence would be confusing and should be avoided. These records must have the standard FITS length of 23040 bits, to make it easy for a FITS reader to go through them in a search for the end of file. A generalized FITS reader should be prepared for them.